The present invention relates to a polishing slurry which is suitably used in a technique for producing a semiconductor device and a method of polishing a substrate by use of the polishing slurry.
In a current production process of a ULSI semiconductor device, processing techniques for achieving a higher density and a higher degree of integration are under study and development. As one of such processing techniques, a CMP (Chemical Mechanical Polishing) technique has been gaining ground as a technique absolutely required for flattening of an interlayer insulating film, formation of shallow trench isolation, and formations of plugs and implanted metal wiring, in a production process of a semiconductor device.
In a conventional production process of a semiconductor device, silica (SiO2)-based particles and ceria (CeO2)-based particles have been widely used as abrasive of a CMP slurry for flattening an inorganic insulating film layer such as a silicon oxide insulating film which is formed by such a method as plasma CVD or low pressure CVD. A representative example of the silica-based abrasive is fumed silica. The fumed silica is produced by dispersing particles which has been grown by such a method as thermal decomposition of silicon tetrachloride, into a medium and then adjusting the pH of the solution. Polishing of an insulating film layer by use of a fumed silica abrasive has a problem that a polishing rate is low.
A representative example of the ceria-based abrasive is cerium oxide. The most outstanding feature of a cerium oxide abrasive is a high polishing rate which cannot be achieved by the silica-based abrasive. A ceria-based compound having a high valence such as cerium oxide, as known as a strong oxidizing agent, has a characteristic of being chemically active. Thus, its chemical action as an oxidizing agent and a mechanical removing action of particles interact with each other. It is believed that the cerium oxide abrasive thereby exhibits the high polishing rate.
The cerium oxide particles are lower in hardness than silica particles or alumina particles and therefore hardly make scratches on a surface to be polished. Hence, it is useful for giving a mirror finish to a surface to be polished. The cerium oxide abrasive is used for, for example, polishing a glass surface.
Taking advantage of these characteristics, the cerium oxide abrasive has been becoming widely used as a CMP abrasive for an insulating film of a semiconductor. This technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 270402-1997. In recent years, along with an increase in the number of layers constituting a semiconductor device and an increase in the degree of integration of the semiconductor device, further increases in a yield and throughput of the semiconductor device have been increasingly demanded. Along with that, faster polishing which causes no scratches has been increasingly desired in a CMP process.
However, if the cerium oxide abrasive for polishing a glass surface is used for polishing an insulating film of a semiconductor as it is, particle diameters of its primary particles are so large that visibly observable scratches are made on the surface of the insulating film due to the large primary particle diameter when the surface is polished so vigorously as to attain a sufficiently high polishing rate. A decrease in the particle diameter of the primary particle can decrease the scratches but also lowers the polishing rate at the same time. In the case of cerium oxide, it is believed that the processing is caused to proceed by its chemical action and a mechanical removing action by particles, and the mechanical removing action by the particles causes scratches.
Thus, as a method for further decreasing scratches in a CMP process using the cerium oxide abrasive, an abrasive improving method such as a selection of the concentration or density or particle diameters of primary particles of the abrasive to cause a desired polishing rate and a surface condition without scratches, or a process improving method such as a reduction in a polishing pressure or a decrease in the rotation speed of a surface plate may be used. However, in any of these methods, there arises a problem that a polishing rate lowers, and it has been considered difficult to achieve a further increase in the polishing rate and a further decrease in the occurrence of the scratches at the same time. In the future, as the number of layers constituting a semiconductor device and the degree of integration of the semiconductor device are further increased, a polishing slurry which causes no scratches and can achieve fast polishing will be absolutely required to improve a yield of the semiconductor device.
Recently, not only to attain a high polishing rate but also to facilitate shallow trench isolation, a polishing slurry with a large ratio between a polishing rate for a silicon oxide insulating film and a polishing rate for a silicon nitride insulating film is demanded. Further, there has been a problem that the pH of a polishing slurry changes with time during polishing or storage and the change in the pH lowers a polishing rate.
In addition, there has been a case where abrasive dispersed in a polishing slurry exhibit instable dispersibility by settling or agglomeration. In evaluating such dispersion stability of the polishing slurry, the dispersion stability has been difficult to evaluate in numerics since particle diameters of the dispersed particles are small and a degree of settling cannot be recognized visually.
The present invention provides a polishing slurry whose abrasives, together with a surface to be polished such as a silicon oxide insulating film, form a chemical reaction layer and which is capable of polishing the layer while achieving an increase in polishing rate and a reduction in scratches at the same time, and a method of polishing a substrate by use of the polishing slurry.
Further, the present invention provides a polishing slurry which can achieve fast polishing with good reproducibility by suppression of a change in its pH with time, has a large ratio between a polishing rate for a silicon oxide film and a polishing rate for a silicon nitride film, has particles favorably dispersed therein so as to reduce occurrences of settling and agglomeration, and has dispersibility detectable and controllable by a variety of optics, and a method of polishing a substrate by use of the polishing slurry.
In view of the foregoing problems, the present inventors have paid attention to polishing which takes advantage of a chemical reaction of particles and is performed with a mechanical action thereof minimized so as to eliminate scratches caused by the particles in a polishing slurry and have made intensive studies, thereby completing the present invention.
A first invention of the present invention is directed to a polishing slurry comprising particles and a medium in which at least a part of the particles are dispersed, wherein the particles are made of at least one of a cerium compound selected from cerium oxide, cerium halide and cerium sulfide and having a density of 3 to 6 g/cm3 and an average particle diameter of secondary particles of 1 to 300 nm, and a tetravalent metal hydroxide.
In particular, the particles preferably have a specific surface area of not smaller than 50 m2/g and an average particle diameter of primary particles of not larger than 50 nm.
In the case where the particles are made of a tetravalent metal hydroxide, it is preferred that the particles have an average particle diameter of secondary particles of not larger than 300 nm, a density of 3 to 6 g/cm3, and an average particle diameter of secondary particles of 1 to 300 nm, that the particles be at least one of a rare earth metal hydroxide and zirconium hydroxide, that the hydroxide of a rare earth metal be cerium hydroxide, and that the particles be a tetravalent metal hydroxide which is obtained by mixing a tetravalent metal salt with an alkali solution.
Further, it is also preferred that the polishing slurry have a pH of 3 to 9, its medium be water, and the polishing slurry contain a pH stabilizer, a dispersant and a polished surface treating agent.
The pH stabilizer preferably comprises one or more constituents, and at least one of the constituents preferably has a pKa value which falls within a 1.0 unit from the pH of the polishing slurry.
The dispersant is preferably selected from a water-soluble anionic dispersant, a water-soluble cationic dispersant, a water-soluble nonionic dispersant and a water-soluble amphoteric dispersant.
The polished surface treating agent is preferably a compound containing at least one atom having an unpaired electron in a molecular structure or a compound containing at least one of a nitrogen atom and an oxygen atom in a molecular structure.
Further, a ratio between a rate at which a silicon oxide insulating film is polished by the polishing slurry and a rate at which a silicon nitride insulating film is polished by the polishing slurry is preferably at least 5.
Still further, the polishing slurry preferably shows a light transmittance of not lower than 10% for light with a wavelength of 500 nm when the particles are contained in an amount of 0.2% by weight, and a difference between light transmittance at a wavelength of 500 to 700 nm of the polishing slurry immediately after its preparation and light transmittance at a wavelength of 500 to 700 nm of the polishing slurry after it is left to stand for 24 hours is preferably not higher than 20%.
Further, the polishing slurry preferably has a conductivity of not higher than 30 mS/cm, and the particles preferably have a positive zeta potential.
A second invention of the present invention relates to a method of polishing a substrate by use of the polishing slurry in the first invention.
In particular, it is preferred that the substrate be polished with a polishing pad having a Shore D hardness of not smaller than 50, the substrate be a substrate in a production process of a semiconductor device, and a silicon oxide film formed on the substrate be polished.
Further, it is preferred that the surface to be polished of the substrate in which at least the silicon oxide insulating film is formed and the polishing pad be moved relatively to each other while the polishing slurry is supplied to the polishing pad on a polishing surface plate so as to polish the substrate, and the silicon oxide insulating film is preferably polished with a polishing slurry containing particles of a tetravalent metal hydroxide such that a polishing rate would be 200 to 2,000 nm/min.
According to the foregoing polishing slurry and substrate polishing method according to the present invention, a surface to be polished such as a silicon oxide insulating film can be polished quickly without scratching the surface. Further, a polishing rate for a silicon oxide insulating film can be made sufficiently larger than a polishing rate for a silicon nitride insulating film so as to selectively polish the silicon oxide insulating film and facilitate control of a process such as shallow trench isolation. In addition, a polishing slurry whose particles show good dispersibility can be obtained, dispersion stability of the polishing slurry can be evaluated in numerics, and the end of polishing and concentrations and particle diameters of the particles in the polishing slurry can be detected and controlled by a variety of optical techniques.